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Stop system select from croaking on read-only COW ""
System select (select with 4 arguments) does not allow any of its
first three arguments to be read-only unless they are undef or
empty strings.
It does not work properly for read-only copy-on-write empty strings,
because it passes all copy-on-write through sv_force_normal under the
expectation that they will shortly be modified. It should not be
doing that for read-only empty strings.